Backup question. What's CH02 or CH03...

I'm trying to make a backup in recovery before flashing, as usual. It's taking forever. Any idea what files like CH02_12_05_02_01_03.jpg could be? There's apparently a million of 'em. The file name changes for a period lasting longer than my determination to let this thing finish. I've poked through my files and I can't find anything like it. Any clue what these jpgs are?